Alkaline batteries, which are a kind of electrochemical device, include a battery case, a positive electrode mixture (positive electrode), a gelled negative electrode containing zinc or a zinc alloy, a separator that separates the positive electrode from the gelled negative electrode, and an alkaline electrolyte.
Recently, the load of devices powered by alkaline batteries has been increasing. Accordingly, there is a demand for alkaline batteries with excellent discharge characteristics. A method that has been employed to improve discharge characteristics of a battery is to mix nickel oxyhydroxide into the positive electrode mixture in order to heighten the potential of the positive electrode.
The battery case of such an alkaline battery is usually made of a steel plate having a nickel plating layer. The inner surface of the battery case almost inevitably has areas where Fe is slightly exposed. Suppressing the formation of such Fe exposed areas is difficult particularly when the thickness of the nickel plating layer is reduced to cut costs.
An alkaline battery is fabricated by placing the positive electrode, the separator, the negative electrode, the alkaline electrolyte, etc. into the battery case and sealing the opening of the battery case. If the inner surface of the battery case has Fe exposed areas, the Fe gradually dissolves therefrom after the battery fabrication, thereby forming an oxide coating, which may partially cover the inner surface of the battery case. As a result, the conductivity inside the battery lowers and the internal resistance increases, so that the discharge characteristics degrade when the battery is stored for a period of time after the battery fabrication (i.e., degradation of storage characteristics, particularly high-rate discharge characteristics).
Also, the Fe ions dissolved from the Fe exposed areas may move in the battery and be deposited on the zinc surface of the negative electrode to form a local cell. As a result, hydrogen gas is produced on the zinc surface, and the produced hydrogen gas reduces the manganese dioxide and nickel oxyhydroxide (positive electrode active materials) thereby decreasing the battery capacity and causing the discharge characteristics to degrade when the battery is stored for a period of time after the battery fabrication (i.e., degradation of storage characteristics, particularly high-rate and low-rate discharge characteristics). Further, if the produced gas builds up, the pressure inside the battery (inner pressure) rises thereby actuating the explosion-proof valve to release the gas for preventing rupture which may result in leakage (degradation of leakage-resistance characteristics).